In connection with the manufacture of endless power transmission belts, such as V-belts, gear belts, and the like, it is conventional to utilize a construction of fabric reinforced rubber or other elastomer. Typically, the belt structure includes a cord winding layer, which is located along the "neutral" axis of the belt. Cross sectional areas of the belts above and below the neutral axis are subject to flexing in tension and/or compression. In a typical endless belt, for example, outer portions of the belt cross section are flexed in tension, as the belt passes around the sheaves over which it is trained. The inner cross sectional portion of the belt are subjected to flexing in compression. For some installations, a single belt may be required to pass over both internal and external sheaves, such that both the inner and outer cross sectional portions thereof are flexed in both tension and compression.
In a transmission belt of conventional construction, strips of reinforcing fabric are incorporated into the elastomeric material of the belt, in the inner and outer cross sectional areas thereof, to impart stability to the belt geometry. Since these areas of the cross section undergo flexing in tension and/or compression, it has been customary to utilize bias-type fabric for the reinforcing material, so that the yarns of the fabric do not extend parallel to the longitudinal axis of the belt, but are arranged at angles thereto. Such reinforcing fabrics can be both wrapped around the exterior of the belt or incorporated internally thereof.
Pursuant to prior practice, the manufacture of bias-type reinforcing fabric has been extremely labor intensive, and therefore costly, and at the same time less than optimally effective for the purpose. In one commonly used procedure, for example, tubular woven fabric is slit along a spiral at an angle of about 75.degree. to the longitudinal axis of the fabric tube. The continuous strip resulting from the bias slitting of the tube has its yarns disposed asymmetrically with respect to the longitudinal axis of the fabric strip. Accordingly, the slit fabric strip is thereafter asymmetrically elongated to reorient the yarns somewhat, in an effort to bring them closer to symmetrical angles of about 30.degree. on either side of the longitudinal axis. The length of slit fabric is impregnated with uncured elastomeric material and then banner cut into short sections, at an angle of about 60.degree. to the axis of the slit fabric. The banner cutting results in a series of parallelogram-shaped sections. These are rotated 90.degree. and then spliced by overlapping. This results in a spliced length of material, in which the yarns are arranged at an angle of around 120.degree., symmetrical to the longitudinal axis of the spliced length. This assembled length is later slit into narrower lengths, appropriate to the desired end use.
Because of crowding of the yarns at the edge extremities of the tubular woven material, a continuous length of the bias cut fabric has periodic diagonal non-uniformities. If allowed to remain in the fabric, these can result in undesirable puckering, bagging or wrinkling. Accordingly, it is often necessary to cut away and discard these sections before banner cutting.
It will be readily apparent from the above that the manufacture and installation of conventional reinforcing materials is a significant labor factor in the manufacture of a power transmission belt. Bias fabrics manufactured in conventional ways have several additional inherent disadvantages. For one, the yarn angles, which ideally are normally about 120.degree. (60.degree. to the longitudinal axis), are very difficult to obtain and control with conventional fabrics, which are woven at 90.degree. and are distorted, by stretching, banner cutting, turning and splicing, in an effort to achieve a symmetrical 120.degree. yarn angle. Precision in achieving the desired angle is difficult, and maintaining the angle during the manufacturing process is also difficult. Additionally, the structure of conventional fabric inherently involves interlacing of the yarns, creating areas of excess wear at the crossover points. Further, the weaving process requires the yarn to have a strength greater than that which is required of the yarn in order to perform its reinforcing function. This requires a higher than necessary level of twist in the yarn, which in itself reduces flexibility of the fabric.
The present invention relates to a novel and significantly improved reinforcing fabric, useful particularly for reinforcement of power transmission belts, hoses and the like. The fabric of the invention is constructed in the first instance with its principal yarns disposed at an optimum wide angle, such as 120.degree.. This of course eliminates the need for the labor-intensive procedures of bias cutting, stretching, banner cutting, turning and splicing the fabric before use. Also in accordance with the invention, the fabric is constructed with its principal yarns laid one over the other, but not interlaced, so that the usual areas of rapid wear, which are inherent in conventional fabrics, are eliminated.
In accordance with one aspect of the invention, the reinforcing fabric is constructed on a special Raschel machine, wherein the principal bias yarns are secured at certain crossover points by longitudinally extending lines knitted stitches, sometimes referred to as pillar stitches, spaced widthwise across the fabric and extending longitudinally thereof. The longitudinal stitching desirably is a lightweight yarn, sufficient merely to maintain the integrity of the fabric during manufacture and assembly.
To particular advantage, the stitching yarns can be formed of a material of a low melting point, which can melt away and dissolve during the curing or vulcanizing procedures which form part of the belt manufacturing operation. As a result, there is minimum restraint of one set of bias yarns by the other, so that each is free to perform its reinforcing function in an optimum manner.
The straight yarn fabric structure of the invention provides a greatly superior reinforcing fabric for use in the manufacture of power transmission belts, hoses and the like. At the same time, the fabric structure of the invention eliminates significant manufacturing steps and other cost factors which are involved in the production of conventional reinforcing fabrics.
Among the important advantages of the new fabric structure is its highly superior flexibility. Greater flexibility has been the subject of decades of research and development in the power transmission belt industry, but has always been restricted by the need for having certain yarn characteristics and certain fabric characteristics dictated largely by the fabric manufacturing process. The fabric of the present invention, on the other hand, enables the yarn and fabric characteristics to be optimized especially for the desired end use, rather than the limitations of the manufacturing process. In this respect, the new fabric represents a quantum improvement over reinforcing fabrics of known construction.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment of the invention and to the accompanying drawings.